Using calcite deformation twins to constrain subduction-related paleostress state and deformation temperatures: A case study using the exhumed Sestola-Vidiciatico unit of the Northern Apennines, Italy

The Sestola-Vidiciatico unit (SVU) in the Northern Apennines, Italy, is an exhumed subduction interface with exposures that reached 200 °C, corresponding to ∼9 km paleodepth. This unit experienced a relatively limited deformation history and serves as a rare analog to the shallowest sedimentary portions of active subduction megathrusts. We use the SVU as a case study of calcite twin analytical methods using calcite shear veins along mineralized faults surrounding the exhumed subduction interface. We reconstruct paleostress orientations through calcite twin stress inversion and use calcite twin paleopiezometry and geothermometry to reconstruct the stress state of the SVU during subduction and subsequent exhumation. We find that stress orientations are consistent between different faults using calcite twin stress inversion and calcite twin geothermometry yield consistent results with organic matter pyrolysis and vitrinite reflectance data, indicating these techniques work well for estimating paleostress orientations and temperatures. Furthermore, our findings are consistent with theoretical and geophysical studies for these tectonic regimes, whereby N- to NE-directed compression during subduction was followed by an inversion of stress orientations during exhumation. However, three of four tested paleopiezometers yield unreasonably high differential stresses, which we ascribe to averaging across a polyphase deformation history as well as differences between the laboratory experiments used to create the piezometers and natural deformation. In addition, calcite twin paleopiezometry and geothermometry suggest the inversion of principal stresses may have coincided with higher differential stresses during exhumation, and we consider possible explanations for this scenario including contrasting mechanical strength and changes in pore fluid pressures.